Bivalve molluscs present particular difficulties for processors for reasons which are based on a number of factors. After bivalve molluscs are harvested from the sea and before they are cooked, they must generally be cleaned. For some bivalve molluscs, the process of removing byssus (or "debyssing") is also involved if the bivalve molluscs are to be acceptable to consumers. When bivalve molluscs have been removed from sea water, their storage life is limited, particularly after they have been debyssed. It is difficult to keep bivalve molluscs alive in a frozen or chilled state, but at the same time the bivalve molluscs must be kept alive until they are cooked if a high quality cooked product is to be obtained.
Accordingly, methods have been proposed for cooking and subsequently freezing the bivalve molluscs as soon as they have been removed from sea water (or shortly afterwards). When bivalve molluscs are heated, however, their shells open and their internal juices are lost. Since most of the taste and flavour of the bivalve molluscs is attributable to the internal juice, this presents a major disadvantage in cooking bivalve molluscs. It is highly preferred to cook the bivalve molluscs while maintaining the shell in a closed state so that the meat is cooked in the juices within the shell. The problems resulting from the opening of the shells are exacerbated because not only is the internal juice lost, but also the cooking medium robs the meat of much of its own flavour (bivalve molluscs are generally cooked in water or steam).
EP-A-0 242 183 describes a process for packing and treating mussels or other bivalve seafood wherein the bivalves are arranged in one or more layers in a heat-shrinkable plastics envelope. The bivalves are vacuum packed within the envelope and the envelope is preferably heat shrunk during vacuum packing. Thus, the bivalves are sealed within a plastics envelope in a closed state which allows them to be heated for partial or total cooking, followed by freezing. This process is disadvantageous because of the necessity of arranging the mussels in one or more layers within a plastics envelope or between two sheets of plastics material, and of carrying out the additional steps of vacuum packing and heat sealing the envelope or plastics sheets with the mussels inside.
Because the mussels are provided to the consumer vacuum packed in the envelopes within which they were cooked, envelopes must be prepared with individual portions of mussels inside, which slows down the process considerably and which leads to additional problems when the method is used in a mass processing facility.
EP-A-0 094 362 describes a process in which mussels are provided in a container with perforated walls and having internal perforated tubes. The container has a lid which can be screwed downwards in order to compact the mussels and inhibit them from opening. This container is placed in a pressurised cooking vessel (e.g. an autoclave) and the mussels are cooked in water or juice. The cooked mussels are frozen within the container and then removed. The juice in which the mussels were cooked is preferably frozen and supplied with the mussels as flavouring or cooking liquor (or it can be used in the preparation of mussel soup).
Although this method appears to more suitable for bulk processing of mussels than the method described in EP-A-0 242 183, the compaction achieved by screwing the lid onto the container is not sufficient to prevent the mussels from opening. This is recognised in the specification of EP-A-0 094 364, which states that the lid serves the dual purpose of partially preventing the opening of the mussels, and of immersing the mussels below the cooking water or juice.
In any event, the fact that juice is lost from the shells is considered in the specification to be compensated for by retaining and freezing the cooking juice (which includes the internal juices of the mussels after cooking) and supplying the frozen cooking juice with the frozen mussels for use in reheating or further cooking the mussels, to give back the lost flavour. Thus, it is stated that "the processing juices are reserved which means that no flavour is lost." Nevertheless, the fact that the shells are allowed to open partially means that the cooking medium robs the meat of some of its flavour. By providing juice or flavoured water as the original cooking liquor, this effect is reduced somewhat, but if the juice is not retained and frozen for supply with the mussels, then this flavour is lost completely. In any event, the organoleptic properties of the mussels will suffer if both the internal juices and the meat flavour are lost during the cooking process, irrespective of what cooking medium is used by the final consumer in the reheating or further cooking of the mussels before consumption.
Thus, it is an object of the invention to provide a process in which bivalve molluscs can be processed in bulk quantities and during which the shells remain closed. It is another object to provide a simpler and cheaper process than the processes known in the prior art, and it is yet a further object of the invention to provide bivalve molluscs having better organoleptic qualities than those produced by known processing methods.
Another object of the invention is the production of a batch of frozen cooked bivalve molluscs which are separated from one another. The problem to be overcome is that bivalve molluscs, when cooked in bulk in water or steam, tend to stick together when immediately frozen thereafter, due to the water retained on the surface of the shells.
Further advantages and/or objects of the invention will become clear by the following description thereof.